VTube-LASER - "Derived Median Line" Concept (Per ASME Y14.5 - GD&T Specification)

VTube-LASER's tube centerline approach is identical to the GD&T "derived median lines" concept. Therefore, VTube's method of handling tube data is ideal for storing and qualifying tube shapes per the ASME Y14.5 standard.

VTube-STEP and VTube-LASER create tube shapes that use the ASME GD&T "derived median line" concept.

A derived median line in GD&T is a theoretical line derived from the center of a cylindrical or spherical feature, such as a hole, pin, or a tube shape. It represents the axis or centerline of that feature. 

This image from page 12 of the ASME Y14.5.1-2019 specification shows a Derived Median Line tube bend.
These are derived median lines for tubes inside VTube-LASER's viewport.

The derived median line represents the axis or centerline of a tube.

Key characteristics of a derived median line:

• It's not a physical feature you can directly measure, but rather a theoretical construct derived from the actual feature.
• For cylindrical tubes, the axis runs through the center of the cylinder.
• For extruded profile tubes that are not cylindrical, the median line can run through an arbitrarily selected feature on the part that runs along the entire length, like a corner of the profile.
• It is established by the center points of circular cross-sections taken perpendicular to the feature's axis.
• It serves as a basis for applying geometric controls like position, straightness, or parallelism to the axis of the feature.

For example, when you apply a position tolerance to a tube in VTube-LASER, you're qualifying the location of the derived median line (axis) of the tube relative to the specified datums. The actual tube often has variation in form, but the derived median line represents the best-fit centerline through that imperfect cylindrical feature of any given tube.

This concept is important because many GD&T controls reference the centerlines or axes of features rather than the physical surfaces themselves.